Radio communication transmitters operate by varying the magnitude and/or phase of a radio frequency (RF) carrier according to an information signal and radiating the modified RF carrier from an antenna. The process of translating information into the magnitude and phase of the RF carrier is known as modulation.
There are various types of modulators that may be used to modulate a digital information stream onto an RF carrier. One commonly used type is the quadrature modulator. FIG. 1 is a block diagram of a prior art quadrature modulator 10. The quadrature modulator 10 comprises an I-channel mixer 100, a Q-channel mixer 102, a local oscillator (LO) 104, a phase shifter 106 and a summer 108. The I-channel mixer 100 is configured to receive an in-phase data stream and a radio frequency (RF) carrier signal from the LO 104. At the same time, the Q-channel mixer 102 is configured to receive a quadrature-phase data stream and a ninety-degree phase shifted version of the carrier signal, by operation of the ninety-degree phase shifter 106. The I- and Q-channel mixers 100 and 102 upconvert the in-phase and quadrature-phase data streams to the frequency of the RF carrier. The summer 108 combines the upconverted in-phase and quadrature-phase signals and feeds the sum to an input of an RF power amplifier (RFPA) 110. The RFPA 110 amplifies the upconverted sum and feeds the upconverted sum to an antenna 112, which radiates the modulated RF carrier for reception by an RF receiver.
Another type of modulator is the polar modulator. A polar modulator processes the modulating signals in polar coordinates, rather than rectangular coordinates as in the quadrature modulator. The ability to process the data in polar, rather than rectangular, coordinates affords the polar modulator several performance advantages over the quadrature modulator. Some of these advantage include higher efficiency, wider dynamic range, less complex and expensive implementation of multi-mode capabilities, enhanced spectral purity and lower susceptibility to temperature variations.
FIG. 2 is a block diagram showing the principle components of a typical prior art polar modulator 20. The polar modulator 20 receives I and Q data streams from a baseband modulator 200. A rectangular-to-polar converter 202 receives the I and an Q data streams and generates an envelope signal containing amplitude information of the input signal and a constant-amplitude phase change signal containing phase information of the input signal. The envelope signal is then processed in an amplitude path of the modulator 20, while the phase change signal is processed separately in a phase path of the modulator 20.
The amplitude path of the polar modulator 20 includes an amplitude-path digital-to-analog converter (DAC) 204 and an amplitude modulator 206. The phase path of the polar modulator 20 includes a phase-path DAC 208 and a voltage controlled oscillator (VCO) 210. The output of the amplitude modulator 206 in the amplitude path of the modulator is coupled to the power input of an RFPA 212, and the output of the VCO 210 is coupled to an RF input of the RFPA 212.
During operation, the phase-path DAC 208 is configured to receive the phase change signal from the rectangular-to-polar converter 430, and convert the phase change signal into an analog signal. The VCO 210 responds to the analog signal by generating a constant-amplitude phase modulated RF drive signal, which is coupled to the RF input of the RFPA 212. Meanwhile, in the amplitude path, the amplitude-path DAC 204 converts the magnitude signal into an analog signal. This analog signal is fed to the amplitude modulator 206, which modulates a power supply voltage (Vsupply), according to the amplitude of the amplitude signal, thereby generating a modulated power supply voltage signal.
The modulated power supply voltage signal from the amplitude path of the polar modulator 20 is coupled to the power input of the RFPA 212, and the RFPA 212, configured for switched mode operation, is driven into heavy compression. The RF output power of the polar modulator 20 is therefore directly proportional to the amplitude signal modulating the power input of the RFPA 212. In addition to the capability of transferring the amplitude information of non-constant envelope modulations such as EDGE (Enhanced Data Rates for GSM (Global System for Mobile Communications) Evolution), the modulated power supply voltage signal also provides accurate power level control.
While the polar modulator has a number of performance advantages over the more conventional quadrature modulator, the magnitude and phase components typically have a much higher bandwidth compared to the in-phase and quadrature-phase components of a quadrature modulator. High bandwidths are undesirable since they increase the rate at which the data must be processed. Higher processing rates require more sophisticated components, which add to the overall cost of the design. Moreover, and notwithstanding the added cost, processing rates are limited by physical limitations of the modulator hardware. So, it is not always an option to avoid high bandwidth related problems simply by providing a more sophisticated design.
In a polar modulator, in particular, the bandwidth of signals in both the amplitude and phase paths must be carefully controlled to ensure proper polar modulator operation. Low-magnitude events in the amplitude path of a polar modulator require that the amplitude path include an amplitude modulator having a large dynamic range. However, it is difficult to design an amplitude modulator having the ability to both provide a large dynamic range, while at the same time providing accurate signal levels.
Low-magnitude events are also susceptible to rapid phase changes. Indeed, for signal magnitudes that pass through zero, the signal phase can change by 180 degrees nearly instantaneously. Rapid phase changes cause the bandwidth of the phase component signal in the phase path of the polar modulator to increase. Unfortunately, high phase bandwidths require high signal processing rates, which either adds to the cost of components in the phase signal path or are not possible with commonly available hardware.
Because low-magnitude events strongly affect signal bandwidth, prior art efforts have focused on ways to reduce or eliminate low-magnitude events in modulators. One prior art approach contemplates “hard limiting” the information signal data, so that low-magnitude events cannot even occur in the first place. Unfortunately, this approach, at least by itself, undesirably leads to substantial spectral regrowth, i.e., a large adjacent channel leakage ratio (ACLR). Moreover, the approach fails to adequately address the phase bandwidth of the signal. As discussed above, rapid changes in phase can significantly increase the phase bandwidth of the signal.
Another approach to limiting low-magnitude events involves injecting pulses having predefined characteristics into the signal stream, at times when it is expected that a low-magnitude event will occur. Such a technique is proposed in U.S. Pat. No. 5,805,640 to O'Dea et al., which describes adding pulses at half-symbol timing instants. A major drawback with that approach, however, is that the signal envelope at T/2 may be greater than a predetermined low-magnitude threshold, but still have a magnitude that actually falls below the threshold at a time or times other than T/2 timing. Consequently, this “T/2 method” is susceptible to entirely missing low-magnitude events.
Yet another approach to reducing low-magnitude events is disclosed in U.S. Pat. No. 5,696,794 to O'Dea et al. In this second O'Dea et al. patent, pulses are added to symbols adjoining a low-magnitude event determined to have occurred at T/2 timing. A significant drawback with that approach, however, is that adding the pulses increases the error vector magnitude (EVM) of the “corrected” symbols. A large EVM is undesirable, since it is an indication that the corresponding symbol being transmitted does not map correctly to the ideal modulation constellation point. Deviation from the ideal constellation point makes it difficult for a receiver to detect the intended symbol and can lead to errors.
Because both O'Dea et al. approaches are limited to symbol rate conditioning, they are prone to error, provide very rough corrections at best and, for many modern applications, simply do not properly address low-magnitude events. Indeed, many modern mobile communications technologies, like UMTS (Universal Mobile Telecommunications Systems) and HSDPA (High-Speed Downlink Packet Access)) display signal trajectories that often have low-magnitude events falling below a predetermined desired threshold. Accordingly, it would not be a safe assumption to simply conclude that low-magnitude events occur only at T/2 timing.
What is needed, therefore, is a method of conditioning low-magnitude events in communications signals that is not limited to T or T/2 timing, does not significantly increase EVM, and which does not unduly degrade other basic signal characteristics such as PSD (Power Spectral Density) and ACLR.